The recent years have seen a rapid expansion of a projectors market and a further market growth is expected. However, to win in the growing market how to deal with a flicker of light from a lamp is becoming an important issue. Brightness has been one of criteria for determining whether a light source for a projector has a superior capability, and a high-pressure mercury-arc lamp developed noting this has a shortest possible arc length and resembles as much as possible a point light source in an effort to enhance the luminance. This gives rise as a side effect to a problem that depending upon the temperature of an electrode of the high-pressure discharge lamp and the condition of a surface of the electrode which is near an arc, a discharge arc develops at an instable location on the electrode and to a phenomenon that the origin of a discharge arc moves from one point to another on the electrode. This phenomenon is visible as a flicker of lamp light (a lamp flicker), and the lamination on a screen irradiated from a projector decreases, which is a major problem in terms also of maintenance of the lamination.
FIG. 13 is a circuitry diagram of a conventional discharge lamp lighting apparatus. The discharge lamp lighting apparatus shown in FIG. 13 includes a d.c. power source section 3 which outputs d.c. voltage Vdc which is obtained by rectifying and smoothing a voltage from a commercial a.c. power source E, a step-down chopper circuit 4 which is connected with an output terminal of the power source section and which provides power control of a discharge lamp La, an invertor circuit 6 which inverts the polarity of a voltage of tile discharge lamp La at a low frequency and which lights up the lamp with a rectangale wave, a discharge lamp current detecting circuit 5 which is formed by a discharge lamp current detecting resistor R1, a discharge lamp voltage detecting circuit 7 which is formed by discharge lamp voltage detecting resistors R4 and R5, and a control circuit block 8 which provides power control.
The discharge lamp voltage detected by the discharge lamp voltage detecting circuit 7 is fed to an A/D conversion input port of a microcomputer 80 which is disposed within the control circuit block 8, and converted into a digital value by a built-in A/D convertor 81. A controller 83 refers to a data table 82, reads power control data Px (X0, X1, . . . , X1023) corresponding to lamp voltage data (0, 1, . . . , 1023) converted into a digital value, and outputs this as a PWM signal. A CR integrating circuit formed by a resistor R6 and a capacitor C2 averages out the PWM signal and transmits this to a PWM control circuit 84 as a reference voltage (command value). The step-down chopper circuit 4 provides the discharge lamp La with electric power which corresponds to an output from the PWM control circuit 84.
An operation of the discharge lamp lighting apparatus shown in FIG. 13 will now be described. FIG. 14 shows the waveform of the d.c. voltage Vdc which is output from the d.c. power source section 3. FIG. 15 shows a discharge lamp current detect voltage and the reference voltage at points A, B and C on the d.c. voltage Vdc. FIG. 16 shows a current IQ1 which flows in a switching element Q1 at the points A, B and C on the d.c. voltage Vdc.
The PWM control circuit 84 detects the current 1Q1 which flows in the switching element Q1 as a voltage across the resistor R1, and when the voltage thus detected exceeds the reference voltage turns off the switching element Q1. Upon turning off of the switching element Q1, a regenerative current of a chopper inductor L1 flows through a diode D1. Owing to a current detected in the diode D1 or a secondary coil output from the inductor L1, the PWM control circuit 84 turns on the switching element Q1 once again upon detection of a zero crossing point of the regenerative current or in accordance with the timing given by an oscillator circuit which is disposed inside the PWM control circuit 84. In this manner, the discharge lamp current is controlled into a current which corresponds to the reference voltage.
However, the d.c, voltage Vdc which is output from the d.c. power source section 3, although smoothed out by the capacitor C1, varies within a range of a few volts to scores of volts (hereinafter referred to as “a ripple”) as shown in FIG. 14. For instance, when the frequency of the commercial a.c. power source is 60 Hz, the ripple frequency of the power source is approximately 120 Hz. In addition, as shown in FIG. 15, the detect voltage becomes slightly higher than the reference voltage because of a delays time t1 (which is from a few ns to a few hundred ns) in the response speed of the PWM control circuit 84 which is disposed inside the control circuit block 8. The excess of the detect voltage at the point A is ΔVA1 over the detect voltage as it is at the point B, and at the point C on the contrary), the detect voltage becomes lower by ΔVC1 than at the point B as shown in FIG. 15. The current IQ1 which flows in the switching element Q1 within the step-down chopper circuit 4 therefore is as shown in FIG. 16 at the respective points A, B and C. This can be explained by the formula IQ1 (peak value)=(Vdc−Vla)×(ON-time of Q1)/L. The symbol Vla denotes the discharge lamp voltage at that time, while the symbol L denotes the inductance value of the inductor L1 inside the step-down chopper circuit 4.
When the discharge lamp La is constant (stably lit), the inductance value L is constant, and therefore, a change of the d.c. voltage Vdc changes the ingredient of the current IQ1 which flows in the switching element Q1. Hence, as shown in FIG. 16, the current IQ1 at the point A becomes higher by ΔIA than at the point B but lower at the point C by ΔIC than at the point B. In consequence, a current ILa flowing through the discharge lamp La is a current which has ripples which are in the same phase as that of Vdc, which causes a control-induced lamp flicker.
Japanese Translation of PCT Internal Application No. 2002-532866, and Japanese Patent Application Laid-Open Gazette No. 2002-134287 disclose a means which reduces a lamp flicker against deterioration of an electrode of a lamp where a method of lighting up with a rectangle wave is used. However, such a means alone can not solve the problem of a control-induced lamp flicker which arises in a discharge lamp lighting circuit itself.